Low-friction and low-adhesion materials and coatings

ABSTRACT

Disclosed are materials that possess both low adhesion and the ability to absorb water. The material passively absorbs water from the atmosphere and then expels this water upon impact with debris, to create a self-cleaning layer. The lubrication reduces friction and surface adhesion of the debris (such as an insect), which may then slide off the surface. The invention provides a material comprising a continuous matrix including a polymer having a low surface energy (less than 50 mJ/m2) and a plurality of inclusions, dispersed within the matrix, each comprising a hygroscopic material. The continuous matrix and the inclusions form a lubricating surface layer in the presence of humidity. The material optionally contains porous nanostructures that inject water back onto the surface after an impact, absorbing water under pressure and then releasing water when the pressure is removed. The material may be a coating or a surface, for example.

PRIORITY DATA

This patent application is a divisional application of U.S. patentapplication Ser. No. 14/658,188, filed on Mar. 14, 2015 (now allowed),which claims priority to U.S. Provisional Patent App. No. 61/953,093,filed on Mar. 14, 2014, each of which is hereby incorporated byreference herein.

FIELD OF THE INVENTION

The present invention generally relates to low-friction, low-adhesionmaterials, coatings, and systems incorporating these.

BACKGROUND OF THE INVENTION

Coatings and materials can become soiled from debris (particles,insects, oils, etc.) impacting the surface. The debris affects airflowover the surface as well as aesthetics and normally is removed bywashing.

Many attempts are described to mitigate insect accumulation during theearly days of aircraft development. These include mechanical scrapers,deflectors, traps, in-flight detachable surfaces, in-flight dissolvablesurfaces, viscous surface fluids, continuous washing fluids, and suctionslots. The results of most of these trials were determined ineffectiveor impractical for commercial use.

Recently, Wohl et al., “Evaluation of commercially available materialsto mitigate insect residue adhesion on wing leading edge surfaces,”Progress in Organic Coatings 76 (2013) 42-50 describe work at NASA tocreate anti-insect adhesion or “bugphobic” surfaces. Wohl et al. testedthe effect of organic-based coatings on insect adhesion to surfaces, butthe results were unsuccessful. Wohl et al. also describe previously usedapproaches to reduce bug adhesion such as mechanical scrapers,deflectors, paper and/or other coverings, elastic surfaces, solublefilms, and washing the surface continually with fluid.

One approach to this problem is to create a self-cleaning surface thatremoves debris from itself by controlling chemical interactions betweenthe debris and the surface.

Superhydrophobic and superoleophobic surfaces create very high contactangles)(>150° between the surface and drops of water and oil,respectively. The high contact angles result in the drops rolling offthe surface rather than remaining on the surface. These surfaces do notrepel solid foreign matter or vapors of contaminants. Once soiled byimpact, debris will remain on the surface and render it ineffective.Also, these surfaces lose function if the nanostructured top surface isscratched.

Fluoropolymer sheets or treated surfaces have low surface energies andthus low adhesion force between foreign matter and the surface. However,friction between impacting debris and the surface results in thesticking of contaminants.

Fluorofluid-filled surfaces have very low adhesion between impactingdebris and the surface. However, if any of the fluid is lost, thesurface cannot be refilled/renewed once applied on the vehicle, and thusloses its properties.

Enzyme-filled coatings leech out enzymes that dissolve debris on thesurface. However, the enzymes are quickly depleted and cannot berefilled, rendering this approach impractical.

Kok et al., “Influence of surface characteristics on insect residueadhesion to aircraft leading edge surfaces,” Progress in OrganicCoatings 76 (2013) 1567-1575, describe various polymer, sol-gel, andsuperhydrophobic coatings tested for reduced insect adhesion afterimpact. The best-performing materials were high-roughness,superhydrophobic surfaces. However, they did not show that debris couldbe removed from the superhydrophobic surfaces once insects broke on thesurface.

In view of the shortcomings in the art, improved materials and materialsystems are needed.

SUMMARY OF THE INVENTION

The present invention addresses the aforementioned needs in the art, aswill now be summarized and then further described in detail below.

In some variations, the invention provides a low-friction, low-adhesionmaterial comprising:

a substantially continuous matrix including a low-surface-energy polymerhaving a surface energy between about 5 mJ/m² to about 50 mJ/m², such asbetween about 10 mJ/m² to about 40 mJ/m²; and

a plurality of inclusions, dispersed within the matrix, each comprisinga hygroscopic material,

wherein the continuous matrix and the inclusions form a lubricatingsurface layer in the presence of humidity.

The material is characterized, according to some embodiments, by a waterabsorption capacity of at least 5 wt % water, preferably at least 10 wt% water, based on total weight of the material.

In some embodiments, the low-surface-energy polymer includes afluoropolymer, such as (but not limited to) a fluoropolymer selectedfrom the group consisting of perfluoroethers, fluoroacrylates,fluorosilicones, and combinations thereof. In these or otherembodiments, the low-surface-energy polymer includes a siloxane.

In some embodiments, the hygroscopic material is selected from the groupconsisting of poly(acrylic acid), poly(ethylene glycol),poly(2-hydroxyethyl methacrylate), poly(vinyl imidazole),poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline),poly(vinylpyrolidone), cellulose, modified cellulose, carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, hydrogels, PEG diacryalate, monoacrylate, and combinationsthereof.

The low-surface-energy polymer and the hygroscopic material may becovalently connected in a block copolymer, in certain embodiments of theinvention.

In some embodiments, the substantially continuous matrix is athree-dimensional template with wells containing the hygroscopicmaterial. In other embodiments, the substantially continuous matrix is athree-dimensional template with pillars (i) containing thelow-surface-energy polymer and (ii) surrounded by the hygroscopicmaterial.

The material may be hydrophobic, i.e., characterized by an effectivecontact angle of water that is greater than 90°. The material may alsobe hydrophilic, i.e. characterized by an effective contact angle ofwater that is less than 90°. The material may also be lipophobic in someof these embodiments.

In some embodiments, the material is characterized by a coefficient offriction, measured at 50% relative humidity, less than 0.9. In these orother embodiments, the material is characterized by a coefficient offriction, measured at 85% relative humidity, less than 0.7.

The material may be characterized by a delay in the formation of ice ona surface of the material.

In various embodiments, the material is a coating and/or is present at asurface of an object or region.

In some variations, the invention provides a low-friction, low-adhesionhydrophobic or hydrophilic material comprising:

a substantially continuous matrix including a low-surface-energy polymerhaving a surface energy between about 5 mJ/m² to about 50 mJ/m²;

a plurality of inclusions, dispersed within the matrix, each comprisinga hygroscopic material; and

a nanoporous material within at least some of the inclusions, whereinthe nanopores have an average pore diameter less than 100 nanometers,wherein the continuous matrix and the inclusions form a lubricatingsurface layer in the presence of humidity.

In some embodiments, the material is characterized by a water absorptioncapacity of at least 5 wt % water based on total weight of the material.

In some embodiments, the low-surface-energy polymer is a polymer fromthe group consisting of perfluoroethers, fluoroacrylates,fluorosilicones, siloxanes, and combinations thereof.

The hygroscopic material may be selected from the group consisting ofpoly(acrylic acid), poly(ethylene glycol), poly(2-hydroxyethylmethacrylate), poly(vinyl imidazole), poly(2-methyl-2-oxazoline),poly(2-ethyl-2-oxazoline), poly(vinylpyrolidone), cellulose, modifiedcellulose, carboxymethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, methyl cellulose, hydrogels, PEG diacryalate,monoacrylate, and combinations thereof, for example.

In some embodiments, the nanoporous material comprises a hydrophobiccomponent and/or is surface-modified to increase hydrophobicity. Thenanoporous material may be selected from the group consisting of silica,alumina, silicates, aluminosilicates, carbonates, carbon, andcombinations thereof, for example.

The material may be characterized by a coefficient of friction, measuredat 50% relative humidity, less than 0.9 and/or a coefficient offriction, measured at 85% relative humidity, less than 0.7. The materialmay be characterized by a delay in the formation of ice on a surface ofthe material.

Some variations provide a precursor material for a low-friction,low-adhesion material, the precursor material comprising:

a hardenable material capable of forming a substantially continuousmatrix, wherein the hardenable material includes a low-surface-energypolymer having a surface energy between about 5 mJ/m² to about 50 mJ/m²;and

a plurality of inclusions, dispersed within the hardenable material,each comprising a hygroscopic material.

In some embodiments of the precursor material, the surface energy of thepolymer is between about 10 mJ/m² to about 40 mJ/m². Thelow-surface-energy polymer may be a fluoropolymer, for example, selectedfrom the group consisting of perfluoroethers, fluoroacrylates,fluorosilicones, and combinations thereof. The low-surface-energypolymer may be a siloxane.

In some embodiments of the precursor material, the hygroscopic materialis selected from the group consisting of poly(acrylic acid),poly(ethylene glycol), poly(2-hydroxyethyl methacrylate), poly(vinylimidazole), poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline),poly(vinylpyrolidone), cellulose, modified cellulose, carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, hydrogels, PEG diacryalate, monoacrylate, and combinationsthereof.

The precursor material may further comprise a nanoporous material withinat least some of the inclusions, wherein the nanopores have an averagepore diameter less than 100 nanometers, and wherein the nanoporousmaterial optionally comprises a hydrophobic component and/or optionallyis surface-modified to increase hydrophobicity. The nanoporous materialmay be selected from the group consisting of silica, alumina, silicates,aluminosilicates, carbonates, carbon, and combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts the structure of some variations of the invention,providing a low-friction, low-adhesion material.

FIG. 1B depicts an exemplary sub-structure of an inclusion with ananomaterial, in some embodiments of the invention.

FIG. 2 illustrates the mode of action according to some variations,showing an insect sliding off the surface following impact.

FIG. 3 shows a phase-contrast AFM image of a through-depth section ofmaterial according to Example 1.

FIG. 4 includes a table of experimental data of friction change as wellas water and oil contact angles, for Example 7.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The materials, compositions, structures, systems, and methods of thepresent invention will be described in detail by reference to variousnon-limiting embodiments.

This description will enable one skilled in the art to make and use theinvention, and it describes several embodiments, adaptations,variations, alternatives, and uses of the invention. These and otherembodiments, features, and advantages of the present invention willbecome more apparent to those skilled in the art when taken withreference to the following detailed description of the invention inconjunction with the accompanying drawings.

As used in this specification and the appended claims, the singularforms “a,” “an,” and “the” include plural referents unless the contextclearly indicates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as is commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs.

Unless otherwise indicated, all numbers expressing conditions,concentrations, dimensions, and so forth used in the specification andclaims are to be understood as being modified in all instances by theterm “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in the following specification andattached claims are approximations that may vary depending at least upona specific analytical technique.

The term “comprising,” which is synonymous with “including,”“containing,” or “characterized by” is inclusive or open-ended and doesnot exclude additional, unrecited elements or method steps. “Comprising”is a term of art used in claim language which means that the named claimelements are essential, but other claim elements may be added and stillform a construct within the scope of the claim.

As used herein, the phrase “consisting of” excludes any element, step,or ingredient not specified in the claim. When the phrase “consists of”(or variations thereof) appears in a clause of the body of a claim,rather than immediately following the preamble, it limits only theelement set forth in that clause; other elements are not excluded fromthe claim as a whole. As used herein, the phrase “consisting essentiallyof” limits the scope of a claim to the specified elements or methodsteps, plus those that do not materially affect the basis and novelcharacteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consistingessentially of,” where one of these three terms is used herein, thepresently disclosed and claimed subject matter may include the use ofeither of the other two terms. Thus in some embodiments not otherwiseexplicitly recited, any instance of “comprising” may be replaced by“consisting of” or, alternatively, by “consisting essentially of.”

Some variations of this invention are premised on the discovery of amaterial that possesses both low surface energy (for low adhesion) andthe ability to absorb water. A hierarchically structured material orcoating, as disclosed, passively absorbs water from the atmosphere andthen expels this water upon impact with the impacting debris, to createa lubrication/self-cleaning layer and reduce the friction and adhesionof the impacting body (such as an insect) on the surface. Because thesematerials trap a layer of water near the surface, they also delay theformation of ice, in some embodiments. The material optionally containsporous nanostructures that inject water back onto the surface after animpact, absorbing water under pressure and then releasing water when thepressure is removed. The material may be used as a coating or as asurface.

In contrast to prior structures and methods, the disclosed material canabsorb water from the air and use this water as a lubricant to wash andremove debris from the surface. The surface contains domains of alow-surface-energy polymer (such as, but not limited to, afluoropolymer) providing low adhesion, and domains of a hygroscopicmaterial that absorbs water and releases it back onto the surface duringimpact. The atmospheric water is thus captured as a lubricant and is acontinually available, renewable resource. The domains of hygroscopicmaterial exist throughout the material, in both planar and depthdimensions. The anti-adhesion function is retained even after abrasionof the top layer of the material.

The disclosed surface reduces the adhesion of debris by releasing waterupon impact. The released water is utilized as a lubricant to reducefriction between debris and the surface. By reducing friction, thedebris is less likely to embed in or otherwise attach to the surface andinstead will slough off the surface (as illustrated in FIG. 2, wheredebris is depicted as a wasp).

Debris may be organic or inorganic and may include insects, dirt, dust,soot, ash, pollutants, particulates, ice, seeds, plant or animalfragments, plant or animal waste products, combinations or derivativesof any of the foregoing, and so on.

Variations of the invention include one or more of three aspects orprinciples. In a first aspect, low-friction and low-adhesion structuresare created by a heterogeneous microstructure comprising alow-surface-energy polymer that is interspersed with hygroscopic domains(lubricating inclusions). Debris impacting the surface has low adhesionenergy with the surface, due to the presence of the low-surface-energypolymer, and the debris will not remain on the surface.

In a second aspect, enhanced friction reduction and cleaning is providedfrom inclusions that collect aqueous lubricant from the environment andexude water upon impact with the surface. The energy of the impactingdebris promotes increased water content at the interface, to act as alubricant between debris and a surface, thus aiding in debris removal.

In a third aspect, nanostructures are contained in the hydroscopicinclusions. The nanostructures contain nanopores that help absorb energy(pressure) of impact, and absorb water during impact. The nanopores thenrebound water to the surface for lubrication and cleaning.

The structure of some variations of the invention is shown in FIGS. 1Aand 1B. FIG. 1A depicts the structure of a coating or surface withlow-friction and self-cleaning properties, and FIG. 1B further depictsenergy-absorption properties.

The structure 100 of FIG. 1 includes a continuous matrix 110. A“continuous matrix” (or equivalently, “substantially continuous matrix”)means that the matrix material is present in a form that includeschemical bonds among molecules of the matrix material. An example ofsuch chemical bonds is crosslinking bonds between polymer chains. In asubstantially continuous matrix 110, there may be present variousdefects, cracks, broken bonds, impurities, additives, and so on.

The structure 100 further includes a plurality of inclusions 120,dispersed within the matrix 110, each of the inclusions 120 comprising ahygroscopic material. An exemplary inclusion 120, in some embodiments,is further depicted in FIG. 1B. The inclusion 120 of FIG. 1B includes ananoporous material 130, wherein nanopores of the nanoporous material130 have an average pore diameter less than 100 nanometers. Thenanoporous material 130 is shown in FIG. 1B (which is not drawn toscale) as being dispersed as multiple nano-inclusions each containingnanoporous material 130. The nanoporous material 130 may also bedispersed more uniformly throughout the inclusion 120, and othergeometric shapes are possible.

Optionally, the continuous matrix 110 may further comprise one or moreadditives selected from the group consisting of fillers, colorants, UVabsorbers, defoamers, plasticizers, viscosity modifiers, densitymodifiers, catalysts, and scavengers.

The mode of action according to some variations is shown in FIG. 2. Thestructure of FIG. 2 includes a continuous matrix 210 and a plurality ofinclusions 220. FIG. 2 illustrates the response of the surface 200 to animpact of debris, which in this illustration is an insect 240 (e.g,wasp), as a non-limiting example. The insect 240 slides across thesurface (200/205) instead of breaking apart, ultimately leaving thesurface 205 and thereby not leaving behind debris bound to the material.

In some variations, the invention provides a low-friction, low-adhesionmaterial comprising:

a substantially continuous matrix including a low-surface-energy polymerhaving a surface energy between about 5 mJ/m² to about 50 mJ/m², such asabout 10, 15, 20, 25, 30, 35, 40, or 45 mJ/m²; and

a plurality of inclusions, dispersed within the matrix, each comprisinga hygroscopic material,

wherein the continuous matrix and the inclusions form a lubricatingsurface layer in the presence of humidity.

A wide range of concentrations of components may be present in thelow-friction, low-adhesion material. For example, the continuous matrixmay be from about 5 wt % to about 95 wt %, such as from about 10 wt % toabout 50 wt % of the material. The hygroscopic inclusions may be fromabout 1 wt % to about 90 wt %, such as from about 10 wt % to about 50 wt% of the coating.

Within the continuous matrix, the low-surface-energy polymer may be fromabout 50 wt % to 100 wt %, such as about 60, 70, 80, 90, 95, or 100 wt%. Within the inclusions, the hygroscopic material may be from about 50wt % to 100 wt %, such as about 60, 70, 80, 90, 95, or 100 wt %.

The material is characterized, according to some embodiments, by a waterabsorption capacity of at least 1, 2, 3, 4, 5, 6, 7, 8, or 9 wt % water,preferably at least 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 wt %water, based on total weight of the material.

As meant herein, a “low-surface-energy polymer” means a polymer, or apolymer-containing material, with a surface energy of no greater than 50mJ/m². The principles of the invention may be applied tolow-surface-energy materials with a surface energy of no greater than 50mJ/m², in general (i.e., not necessarily limited to polymers).

In some embodiments, the low-surface-energy polymer includes afluoropolymer, such as (but not limited to) a fluoropolymer selectedfrom the group consisting of perfluoroethers, fluoroacrylates,fluorosilicones, and combinations thereof.

In these or other embodiments, the low-surface-energy polymer includes asiloxane. A siloxane contains at least one Si—O—Si linkage. Thelow-surface-energy polymer may consist of polymerized siloxanes orpolysiloxanes (also known as silicones). One example ispolydimethylsiloxane.

In some embodiments, the hygroscopic material is selected from the groupconsisting of poly(acrylic acid), poly(ethylene glycol),poly(2-hydroxyethyl methacrylate), poly(vinyl imidazole),poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline),poly(vinylpyrolidone), cellulose, modified cellulose, carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, hydrogels, PEG diacryalate, monoacrylate, and combinationsthereof.

In certain embodiments, the hygroscopic material is also classified as ahydrophilic material. A hygroscopic substance will actively attract andabsorb water, without necessarily bonding. A hydrophilic substance willbond, on a molecular level, with water.

The plurality of inclusions may include 2, 3, 4, 5, 10, 20, 30, 40, 50,75, 100, or more inclusions comprising hygroscopic material within aregion of low-friction, low-adhesion material (see e.g. FIG. 3). Thelow-surface-energy polymer and the hygroscopic material arephase-separated, i.e. they do not form a single continuous phase.

The hygroscopic inclusions are three-dimensional objects or domains,which may be of any shape, geometry, or aspect ratio. In athree-dimensional object, an aspect ratio of exactly 1.0 means that allthree characteristic length scales are identical, such as in a perfectcube. The aspect ratio of a perfect sphere is also 1.0. The hygroscopicinclusions may be geometrically symmetric or asymmetric. Randomly shapedasymmetric templates are, generally speaking, geometrically asymmetric.In some embodiments, hygroscopic inclusions are geometrically symmetric.Examples include cylinders, cones, rectangular prisms, pyramids, orthree-dimensional stars.

In some embodiments, the hygroscopic inclusions are anisotropic. Asmeant herein, “anisotropic” templates have at least one chemical orphysical property that is directionally dependent. When measured alongdifferent axes, an anisotropic template will have some variation in ameasurable property. The property may be physical (e.g., geometrical) orchemical in nature, or both. The property that varies along multipleaxes may simply be the presence of mass; for example, a perfect spherewould be geometrically isotropic while a three-dimensional star shapewould be anisotropic. A chemically anisotropic inclusion may vary incomposition from the surface to the bulk phase, such as via a chemicallymodified surface. The amount of variation of a chemical or physicalproperty, measured along different axes, may be 5%, 10%, 20%, 30%, 40%,50%, 75%, 100% or more.

The hygroscopic inclusions may be characterized as templates, domains,or regions. The hygroscopic inclusions are not a single, continuousframework in the coating. Rather, the hygroscopic inclusions arenon-continuous and dispersed in the continuous matrix. The hygroscopicinclusions are preferably dispersed uniformly within the continuousmatrix.

The hygroscopic inclusions themselves may possess multiple lengthscales. For example, the hygroscopic inclusions may have an averageoverall particle size as well as another length scale associated withporosity, surface area, surface layer, sub-layer, protrusions, or otherphysical features.

The low-surface-energy polymer and the hygroscopic material may becovalently connected in a block copolymer, in certain embodiments of theinvention. In this context, “covalently connected” refers to chemicalbonds within the block copolymer that bond at least a portion of thelow-surface-energy polymer with the hygroscopic material. This polymermay be a flat sheet or a polymer film with 3D texture, such as pillarsor wells.

In embodiments without such a block copolymer, the low-surface-energypolymer and the hygroscopic material are present as a plurality ofinclusions, dispersed within a substantially continuous matrix includinga low-surface-energy polymer. There may be, but is not necessarily, somedegree of chemical and/or physical bonding between thelow-surface-energy polymer and the hygroscopic material.

In some embodiments, the substantially continuous matrix is athree-dimensional template with wells (depressions in thelow-surface-energy polymer continuous matrix) containing the hygroscopicmaterial in the well spaces. In other embodiments, the substantiallycontinuous matrix is a three-dimensional template that includes pillarsmade from, and containing, the low-surface-energy polymer. The pillarsare surrounded by the hygroscopic material. In this type of structure,the plurality of inclusions (domains) can be regarded either as theregions of hygroscopic material surrounding each pillar, or as thehygroscopic material filling the space between adjacent pillars, forexample.

The material may be hydrophobic, i.e., characterized by an effectivecontact angle of water that is greater than 90°. The material may alsobe hydrophilic, i.e. characterized by an effective contact angle ofwater that is less than 90°. In various embodiments, the material ischaracterized by an effective contact angle of water of about 70°, 75°,80°, 85°, 90°, 95°, 100°, or higher.

The material may also be lipophobic or partially lipophobic in someembodiments. In various embodiments, the material is characterized by aneffective contact angle of hexadecane (as a measure of lipophobicity) ofabout 50°, 55°, 60°, 65°, 70°, 75°, 80°, 85°, 90°, or higher.

The material may simultaneously have hydrophobic and lipophobicproperties. In certain embodiments, the material is characterized by aneffective contact angle of water of at least 90° (such as about 95-100°)and simultaneously an effective contact angle of hexadecane of at least60° (such as about 65°). In certain embodiments, the material ischaracterized by an effective contact angle of water of at least 80°(such as about 80-85°) and simultaneously an effective contact angle ofhexadecane of at least 70° (such as about 75-80°).

In some embodiments, the material is characterized by a coefficient offriction, measured at 40-55% (e.g. 50%) relative humidity and roomtemperature, less than 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, or 0.3. In theseor other embodiments, the material is characterized by a coefficient offriction, measured at 85% relative humidity and room temperature, lessthan 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, or 0.2.

The coefficient of friction is relatively low due to the presence of alubricating surface layer. By a “lubricating surface layer in thepresence of humidity,” it is meant a layer, multiple layers, a partiallayer, or an amount of substance that lubricates the substrate such thatit has a lower coefficient of friction compared to the substrate withoutthe material present, when in the presence of some amount of atmospherichumidity.

The specific level of humidity is not regarded as critical, but ingeneral may range from about 1% to 100%, typically about 30% to about70% relative humidity. Relative humidity is the ratio of the water vapordensity (mass per unit volume) to the saturation water vapor density.Relative humidity is also approximately the ratio of the actual to thesaturation vapor pressure.

The substance that lubricates the substrate is primarily water, but itshould be noted that other components may be present in the lubricatingsurface layer, including oils, metals, dust, dissolved gases, dissolvedaqueous components, suspended non-aqueous components, fragments ofdebris, fragments of polymers, and so on.

The material may be characterized by a delay in the formation of ice ona surface of the material. For example, when a material surface is heldat −10° C., the material provided by the invention may be characterizedby a delay in the formation of ice on the surface of at least about 1,2, 3, 4, 5, 10, 15, 20, 25, 30 minutes, or more.

In various embodiments, the material is a coating and/or is present at asurface of an object or region. The material may be utilized inrelatively small applications, such as lens coatings, or for largestructures, such as aircraft wings. In principle, the material could bepresent within a bulk region of an object or part, or could contain atemporary, protective laminating film for storage or transport, which islater removed for use of the material.

The continuous matrix offers durability, impact resistance, and abrasionresistance to the coating. There is homogeneity through the z-directionof the film, so that if some portion of the coating is lost (despite theresistance to abrasion), the remainder retains the desired properties.The coating offers a repeating, self-similar structure that allows thecoating to be abraded during use while retaining properties. Should thesurface be modified due to environmental events or influences, theself-similar nature of the coating allows the freshly exposed surface topresent a coating identical to that which was removed.

In some variations, the invention provides a low-friction, low-adhesionhydrophobic or hydrophilic material comprising:

a substantially continuous matrix including a low-surface-energy polymerhaving a surface energy between about 5 mJ/m² to about 50 mJ/m²;

a plurality of inclusions, dispersed within the matrix, each comprisinga hygroscopic material; and

a nanoporous material within at least some of the inclusions, whereinthe nanopores have an average pore diameter preferably less than 100nanometers (such as less than 90, 80, 70, 60, 50, 40, 30, 20, or 10nanometers),

wherein the continuous matrix and the inclusions form a lubricatingsurface layer in the presence of humidity.

In some embodiments, the material is characterized by a water absorptioncapacity of at least 5, 10, 15, or 20 wt % water based on total weightof the material.

In some embodiments, the low-surface-energy polymer is a polymer fromthe group consisting of perfluoroethers, fluoroacrylates,fluorosilicones, siloxanes, and combinations thereof.

The hygroscopic material may be selected from the group consisting ofpoly(acrylic acid), poly(ethylene glycol), poly(2-hydroxyethylmethacrylate), poly(vinyl imidazole), poly(2-methyl-2-oxazoline),poly(2-ethyl-2-oxazoline), poly(vinylpyrolidone), cellulose, modifiedcellulose, carboxymethyl cellulose, hydroxyethyl cellulose,hydroxypropyl cellulose, methyl cellulose, hydrogels, PEG diacryalate,monoacrylate, and combinations thereof, for example. The water uptake ofvarious polymers is described in J. Mater. Chem. (17) 2007, 4864-4871,which is hereby incorporated by reference herein.

In some embodiments, the nanoporous material comprises a hydrophobiccomponent and/or is surface-modified to increase hydrophobicity. Thenanoporous material may be selected from the group consisting of silica,alumina, silicates, aluminosilicates, carbonates, carbon, andcombinations thereof, for example.

In some embodiments, with reference to FIG. 1B, nanoparticles 130comprise a nanomaterial selected from the group consisting of silica,alumina, titania, zinc oxide, polytetrafluoroethylene, polystyrene,polyurethane, silicones, and combinations thereof. In certainembodiments, the nanoparticles 130 comprise silica. Other nanoparticles130 are possible, as will be appreciated. Optionally, the nanoparticles130 may be surface-modified with a hydrophobic material, such as (butnot limited to) silanes including alkylsilane, fluoroalkylsilane,alkyldisilazane (e.g., hexamethyldisilazane), or poly(dimethylsiloxane).

The nanoporous material preferably contains pores less than 100 nm indiameter. In some embodiments, the nanoporous material is inherentlyhydrophobic. Alternatively, or additionally, the nanopores may be coatedwith a hydrophobic surface treatment.

The nanopores have a capillary pressure p_(c) that depends on the porediameter r, the surface tension of water γ, and the contact angle ofwater on the pore surface θ.

$p_{c} = \frac{2\gamma \; \cos \; \theta}{r}$

The capillary pressure p_(c) will push against water infiltration. Onlyduring impact will the surrounding pressure increase and water will beforced into the pore. Immediately after impact, water is forced out ofthe pores at the capillary pressure and will flood onto the surface tohelp wash away debris. See Kong et al., “Energy absorption of nanoporoussilica particles in aqueous solutions of sodium chloride,” Phys. Scr. 74(2006) 531-534, which is hereby incorporated by reference herein, forits teachings regarding nanoporous energy absorbers which may beemployed for water absorption and rejection. The impact pressure fromdebris on the surface of the material may be calculated using AppendixA1 in Jilbert and Field, “Synergistic effects of rain and sand erosion,”Wear 243 (2000) 6-17, which is hereby incorporated by reference herein.

When nanoparticles are dispersed within the continuous matrix, thenanoparticles may be either located within the hygroscopic inclusions orseparately dispersed within the continuous matrix (or both of these).The nanoparticles preferably have a length scale from about 5 nm toabout 100 nm, such as about 10 nm to about 50 nm. Here, a nanoparticlelength scale refers for example to a diameter of a sphere, a height orwidth of a rectangle, a height or diameter of a cylinder, a length of acube, an effective diameter of a nanoparticle with arbitrary shape, andso on.

The nanoparticles may be chemically and/or physically bonded to, locatedwithin, or otherwise associated with, the hygroscopic inclusions.Alternatively, or additionally, the nanoparticles may be disperseduniformly within the continuous matrix but not necessarily directlyassociated with the hygroscopic inclusions.

A wide range of concentrations of components may be present in thematerial. For example, the continuous matrix may be from about 5 wt % toabout 95 wt %, such as from about 10 wt % to about 50 wt % of thematerial. The hygroscopic inclusions may be from about 1 wt % to about90 wt %, such as from about 10 wt % to about 50 wt % of the coating. Thenanoparticles may be from about 0.1 wt % to about 25 wt %, such as fromabout 1 wt % to about 10 wt % of the material.

Within the continuous matrix, the low-surface-energy polymer may be fromabout 50 wt % to 100 wt %, such as about 60, 70, 80, 90, 95, or 100 wt%. Within the inclusions, the hygroscopic material may be from about 50wt % to 100 wt %, such as about 60, 70, 80, 90, 95, or 100 wt %, notincluding the nanomaterial.

The material may be characterized by a coefficient of friction, measuredat 50% relative humidity, less than 0.9 and/or a coefficient offriction, measured at 85% relative humidity, less than 0.7. The materialmay be characterized by a delay in the formation of ice on a surface ofthe material.

In some variations, the invention provides a low-friction, low-adhesionmaterial comprising:

a substantially continuous matrix including a low-surface-energy polymerhaving a surface energy between about 10 mJ/m² to about 40 mJ/m²;

a plurality of inclusions, dispersed within the matrix, each comprisinga hygroscopic material; and

optionally a nanoporous material within at least some of the inclusions,wherein the nanopores have an average pore diameter less than 100nanometers,

wherein the continuous matrix and the inclusions support the formationof a lubricating surface layer.

In some embodiments, the material further includes voids. As intendedherein, a “void” is a discrete region of empty space, or space filledwith air or another gas, that is enclosed within the continuous matrix.The voids may be open (e.g., interconnected voids) or closed (isolatedwithin the continuous matrix), or a combination thereof. The voids maypartially surround inclusions or nanoparticles.

In various embodiments, the material is a coating and/or is present at asurface of an object or region.

Any known methods to fabricate these materials or coatings may beemployed. Notably, these materials or coatings may utilize synthesismethods that enable simultaneous deposition of components or precursormaterials to reduce fabrication cost and time. In particular, thesematerials or coatings may be formed by a one-step process, in someembodiments. In other embodiments, these materials or coatings may beformed by a multiple-step process.

The low-friction, low-adhesion hydrophobic or hydrophilic material, insome embodiments, is formed from a precursor material (or combination ofmaterials) that may be provided, obtained, or fabricated from startingcomponents. The precursor material is capable of hardening or curing insome fashion, to form a substantially continuous matrix including alow-surface-energy polymer along with a plurality of hygroscopicinclusions, dispersed within the matrix, and optionally a nanoporousmaterial within at least some of the inclusions. The precursor materialmay be a liquid; a multiphase liquid; a multiphase slurry, emulsion, orsuspension; a gel; or a dissolved solid (in solvent), for example.

The low-surface-energy polymer and the hygroscopic material may be inthe same phase or in different phases. In some embodiments, thelow-surface-energy polymer is in liquid or dissolved form while thehygroscopic material is in dissolved-solid or suspended solid form. Insome embodiments, the low-surface-energy polymer is dissolved-solid orsuspended-solid form while the hygroscopic material is in liquid ordissolved form. In some embodiments, the low-surface-energy polymer andthe hygroscopic material are both in liquid form. In some embodiments,the low-surface-energy polymer and the hygroscopic material are both indissolved (solvent) form.

Some variations provide a precursor material for a low-friction,low-adhesion material, the precursor material comprising:

a hardenable material capable of forming a substantially continuousmatrix, wherein the hardenable material includes a low-surface-energypolymer having a surface energy between about 5 mJ/m² to about 50 mJ/m²;and

a plurality of inclusions, dispersed within the hardenable material,each comprising a hygroscopic material.

In some embodiments of the precursor material, the surface energy of thepolymer is between about 10 mJ/m² to about 40 mJ/m². Thelow-surface-energy polymer may be a fluoropolymer, for example, selectedfrom the group consisting of perfluoroethers, fluoroacrylates,fluorosilicones, and combinations thereof. The low-surface-energypolymer may be a siloxane.

In some embodiments of the precursor material, the hygroscopic materialis selected from the group consisting of poly(acrylic acid),poly(ethylene glycol), poly(2-hydroxyethyl methacrylate), poly(vinylimidazole), poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline),poly(vinylpyrolidone), cellulose, modified cellulose, carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, hydrogels, PEG diacryalate, monoacrylate, and combinationsthereof.

In some embodiments, the hygroscopic material is also hardenable, eitheralone or in combination with the low-surface-energy polymer. Forinstance, a low-surface-energy polymer and a hygroscopic polymer mayform a high-molecular-weight block copolymerize and thus harden (seeExample 1). In certain embodiments, the hygroscopic material assists inthe curability (hardenability) of the low-surface-energy polymer.

The precursor material may further comprise a nanoporous material withinat least some of the inclusions, wherein the nanopores have an averagepore diameter less than 100 nanometers, and wherein the nanoporousmaterial optionally comprises a hydrophobic component and/or optionallyis surface-modified to increase hydrophobicity. The nanoporous materialmay be selected from the group consisting of silica, alumina, silicates,aluminosilicates, carbonates, carbon, and combinations thereof. Thenanoporous material is typically already in a solid state orsolid-solution state, within the precursor material. During final curingit is possible for the nanoporous material to undergo additionalchemical curing reactions or other reactions.

In some embodiments, a precursor material is prepared and then dispensed(deposited) over an area of interest. Any known methods to depositprecursor materials may be employed. A fluid precursor material allowsfor convenient dispensing using spray coating or casting techniques overa large area, such as the scale of a vehicle or aircraft.

The fluid precursor material may be applied to a surface using anycoating technique, such as (but not limited to) spray coating, dipcoating, doctor-blade coating, spin coating, air knife coating, curtaincoating, single and multilayer slide coating, gap coating,knife-over-roll coating, metering rod (Meyer bar) coating, reverse rollcoating, rotary screen coating, extrusion coating, casting, or printing.Because relatively simple coating processes may be employed, rather thanlithography or vacuum-based techniques, the fluid precursor material maybe rapidly sprayed or cast in thin layers over large areas (such asmultiple square meters).

When a solvent is present in the fluid precursor material, the solventmay include one or more compounds selected from the group consisting ofalcohols (such as methanol, ethanol, isopropanol, or tert-butanol),ketones (such as acetone, methyl ethyl ketone, or methyl isobutylketone), hydrocarbons (e.g., toluene), acetates (such as tert-butylacetate), organic acids, and any mixtures thereof. When a solvent ispresent, it may be in a concentration of from about 10 wt % to about 99wt % or higher, for example.

When a carrier fluid is present in the fluid precursor material, thecarrier fluid may include one or more compounds selected from the groupconsisting of water, alcohols, ketones, acetates, hydrocarbons, acids,bases, and any mixtures thereof. When a carrier fluid is present, it maybe in a concentration of from about 10 wt % to about 99 wt % or higher,for example.

The precursor material may be converted to an intermediate material orthe final material using any one or more of curing or other chemicalreactions, or separations such as removal of solvent, monomer, water, orvapor. Curing refers to toughening or hardening of a polymeric materialby cross-linking of polymer chains, assisted by electromagnetic waves,electron beams, heat, and/or chemical additives. Chemical removal may beaccomplished by heating/flashing, vacuum extraction, solvent extraction,centrifugation, etc. Physical transformations may also be involved totransfer precursor material into a mold, for example. Additives may beintroduced during the hardening process, if desired, to adjust pH,stability, density, viscosity, color, or other properties, forfunctional, ornamental, safety, or other reasons.

The overall thickness of the final material or coating may be from about1 μm to about 1 cm or more, such as about 10 μm, 20 μm, 25 μm, 30 μm, 40μm, 50 μm, 75 μm, 100 μm, 500 μm, 1 mm, 1 cm, or 10 cm. Relatively thickcoatings offer good durability and mechanical properties, such as impactresistance, while preferably being relatively lightweight.

EXAMPLES Example 1: Hygroscopic-Fluoro Block Polymer

Polyethylene glycol (M_(n)=3400) is dried using a freeze dryer overnightand the powder (2.55 g) is charged to a vial followed by hexamethylenediisocyanate, HMDI (1.97 g). A small PTFE-coated stir bar is introducedand the vial placed in a 100° C. oil bath to stir. Once the polymer hasmelted and dissolved in the HMDI, a small drop of dibutyltin dilaurate(˜10 μL) is added and stirred into the mix. The reaction mixture is leftto stir for 1 hour.

Following this period, Flurolink D4000 pefluoroether (3 g) is injectedinto the mixture and then promptly vortexed to homogenize beforereturning to the 100° C. oil bath. The mixture quickly thickens over thecourse of 5 minutes, where it is vortexed again before being left for 1hour. At the end of this period the vial is taken off the heat and THF(2 mL) added to the viscous resin and vortexed to disperse and thin theoverall mixture (precursor material). While still warm, butanediol (540mg) is dissolved in THF (1 mL) and injected into the precursor materialand promptly vortexed. Shortly after this step, the precursor materialresin is poured into a 3″×3″ PTFE mold to flash off solvent and cure theprecursor material film at room temperature overnight.

The next day the film is placed in an 80° C. oven for 4 hours tocomplete the cure before a portion is flattened on a hot press at 100°C. using a shim of ˜0.5 mm to control film thickness. A phase-contrastAFM (atomic force microscopy) image of a through-depth section of thismaterial film is shown in FIG. 3. The data shows microphase separationof the fluoro and hygroscopic sections of the polymer. In particular,FIG. 3 is an AFM image of Example 1 showing clear phase separation intosoft oblong regions (darker regions in the image) surrounded by acontinuous stiffer matrix.

When exposed to 90% humidity for 30 minutes, this sample increases 25.2wt % in mass, which is indicative of the amount of water absorbed by thehygroscopic component of the polymer.

Example 2: Hygroscopic-Fluoro Block Polymer

Polyethylene glycol (M_(n)=3400) is dried using a freeze dryer overnightand the powder (2.55 g) is charged to a vial followed by hexamethylenediisocyanate, HMDI (1.97 g). A small PTFE-coated stir bar is introducedand the vial placed in a 100° C. oil bath to stir. Once the polymer hasmelted and dissolved in the HMDI, a small drop of dibutyltin dilaurate(˜10 μL) is added and stirred into the mix. The reaction mixture is leftto stir for 1 hour.

Following this period, Flurolink D4000 pefluoroether (3 g) is injectedinto the mixture and then promptly vortexed to homogenize beforereturning to the 100° C. oil bath. The mixture quickly thickens over thecourse of 5 minutes and it is vortexed again before being left for 1hour. At the end of this period, the vial is taken off the heat and THF(2 mL) added to the viscous resin and vortexed to disperse and thin theoverall mixture. While still warm, Jeffamine D230 with ketamine endgroups (2.36 g) is injected into the mixture and promptly vortexed.Shortly after this step, the resin is poured into a 3″×3″ PTFE mold toflash off solvent and cure the film at room temperature overnight.

The next day the film is placed in an 80° C. oven for 4 hours tocomplete the cure. Then a portion is flattened on a hot press at 100° C.using a shim of −0.5 mm to control film thickness.

When exposed to 90% humidity for 30 minutes, this sample increases 26.0wt % in mass, which is indicative of the amount of water absorbed by thehygroscopic component of the polymer.

Example 3: Fluoropolymer Control

Flurolink D4000 pefluoroether (4 g) is charged to a vial followed byHMDI (0.786 g). A small PTFE-coated stir bar is introduced and the vialplaced in a 100° C. oil bath to stir. The reaction is vortexedaggressively after achieving a temperature of 100° C., and then left tostir for 1 hour. After this step, the resin is poured into a 3″×3″ PTFEmold to flash off solvent and cure the film at room temperatureovernight.

Example 4: Textured Hygroscopic-Fluoro Block Polymer

Butanediol is mixed with the Example 1 polymer and vortexed. Then analiquot of the liquid is cast onto the patterned fluropolymer of Example5. A polyethylene sheet is placed onto the stamp to remove excesspolymer. The filled stamp is then allowed to cure at ambient temperatureovernight, placed in an 80° C. oven for 4 hours, and then peeled awayfrom the fluoropolymer mold.

Example 5: Textured Fluoropolymer

A patterned PFPE-DMA (perfluoropolyether dimethacrylate) mold isgenerated by pooling PFPE-DMA containing 1-hydroxycyclohexyl phenylketone over a patterned silicon substrate. A poly(dimethylsiloxane)gasket is used to confine the pooled PFPE-DMA precursor to the desiredarea on the silicon substrate. These mold fabrication materials areplaced in a small UV-curing chamber and the chamber is purged withnitrogen for 5 minutes. Polymerization of the PFPE-DMA precursors isaccomplished by UV photoirradiation (wavelength of 365 nm) for 10minutes while remaining under the nitrogen purge. The fully curedPFPE-DMA elastomeric mold is then released from the silicon master andused for molding applications.

The procedure in this Example 5 may be utilized to form a fluoropolymerwith wells or 3D pillars, for instance, followed by filling the wells(e.g., Example 6), or surrounding the pillars, with a hygroscopicmaterial.

Example 6: PAA-Filled Textured Fluoropolymer

Polyacrylic acid is dissolved in an ethanol/water solution. The mixtureis airbrushed onto the patterned fluropolymer provided by Example 5. Apolyethylene sheet is placed onto the stamp to remove excess PAA. Thefilled stamp is allowed to dry, leaving the PAA in the pores of thepatterned stamp.

Example 7: Contact Angle and Friction Testing

The change in friction in response to humidity is tested byequilibrating the samples of Examples 1-6 at ambient (40-55%) relativehumidity or 90% relative humidity in a humidity-controlled chamber. Thenthe samples are placed on a variable-angle stage and the angle isincreased until a 5-gram cylindrical mass slides along the samplesurface. The sliding angle is used to determine the friction constant(coefficient of friction). The friction change as well as water and oilcontact angles are shown for the samples of Examples 1-6 in the table ofFIG. 4.

Both Example 1 and Example 2 samples have contact angles similar to thepure fluoropolymer control (Example 3); however, they show lower overallfriction, and in the case of Example 1, a greater decrease in frictionwith humidity. This shows that the fluoropolymer block is creating alow-adhesion surface while the hygroscopic block is absorbing water thatcreates overall reduced friction as compared to the fluoropolymercontrol (Example 3).

These materials (Example 1 and Example 2) differ significantly fromtypical hygroscopic materials that would have a very low water contactangle and high friction. The high water contact angle in our materialsmeans that drops will better slide across the surface and createlubrication. The moderate oil (hexadecane) contact angle shows that oilwill not spread over the surface, which would render it ineffective.

The Example 4 sample is the Example 1 polymer formed with an array of 3Dwells in the surface. This texturing results in overall lower frictionthan the Example 1 coating material from which it is made, or a texturedpure fluoropolymer (Example 5).

When the Example 5 textured fluoropolymer is filled with a hygroscopicmaterial (Example 6), it shows a larger decrease in friction withincreasing humidity as compared to Example 5 (34% compared to 22%), dueto water from the air being absorbed into the hydroscopic material andproviding additional lubricity. The hygroscopic material of Example 6also increases the lipophobicity without compromising muchhydrophobicity.

Example 8: Ice Formation Testing

The kinetic delay of freezing is measured by placing three 50 μL dropsof deionized water on a surface held at −10° C. with a thermoelectriccooler. The time for ice to initially form in the droplets is measured.A bare aluminum surface has an ice formation delay of 13±6 seconds. TheExample 3 fluoropolymer control demonstrates an ice formation delay of 1min 18 seconds±53 seconds. The Example 1 hygroscopic-fluoro blockcopolymer demonstrates an ice formation delay of 27 min 43 seconds±41seconds.

The surprisingly long ice formation delay of the Example 1 material maybe due to the material trapping water at the surface. Without beinglimited by theory, it is believed that this trapped layer of watercannot freeze because the hydroscopic domains inhibit thecrystallization reaction mechanisms in the surface water. Any droplet ofwater on the surface sees liquid water instead of a coating on thesurface; ice nucleation is confined to the homogeneous nucleation regimethat is kinetically much slower than heterogeneous nucleation.

Vehicle-based cameras for surrounding awareness will require lenscoatings that will inhibit soiling in order to function. Once soiled,the camera will lose effectiveness and eventually cease functioning. Thecoatings/surfaces described herein may be used as camera lens coatings,and may be transparent.

Aircraft lose efficiency from disruption of laminar flow when insect andparticulate debris collect on the aircraft wings. This inventionprovides materials that reduce the adhesion of insect and particulatedebris on aircraft surfaces, while simultaneously inhibiting theformation of ice.

Other practical applications for the present invention include, but arenot limited to, vehicle windows, optical lenses, filters, instruments,sensors, eyeglasses, cameras, satellites, and weapon systems. Forexample, automotive applications can utilize these coatings to preventthe formation of ice or debris on back-up camera lenses or back-upsensors. The principles taught herein may also be applied toself-cleaning materials, anti-adhesive coatings, corrosion-freecoatings, etc.

In this detailed description, reference has been made to multipleembodiments and to the accompanying drawings in which are shown by wayof illustration specific exemplary embodiments of the invention. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatmodifications to the various disclosed embodiments may be made by askilled artisan.

Where methods and steps described above indicate certain eventsoccurring in certain order, those of ordinary skill in the art willrecognize that the ordering of certain steps may be modified and thatsuch modifications are in accordance with the variations of theinvention. Additionally, certain steps may be performed concurrently ina parallel process when possible, as well as performed sequentially.

All publications, patents, and patent applications cited in thisspecification are herein incorporated by reference in their entirety asif each publication, patent, or patent application were specifically andindividually put forth herein.

The embodiments, variations, and figures described above should providean indication of the utility and versatility of the present invention.Other embodiments that do not provide all of the features and advantagesset forth herein may also be utilized, without departing from the spiritand scope of the present invention. Such modifications and variationsare considered to be within the scope of the invention defined by theclaims.

1. A precursor material for a low-friction, low-adhesion material, saidprecursor material comprising: a hardenable material capable of forminga substantially continuous matrix, wherein said hardenable materialincludes a low-surface-energy polymer having a surface energy betweenabout 5 mJ/m² to about 50 mJ/m²; and a plurality of inclusions,dispersed within said hardenable material, each comprising a hygroscopicmaterial.
 2. The precursor material of claim 1, wherein said surfaceenergy of said polymer is between about 10 mJ/m² to about 40 mJ/m². 3.The precursor material of claim 1, wherein said low-surface-energypolymer is a fluoropolymer.
 4. The precursor material of claim 3,wherein said fluoropolymer is selected from the group consisting ofperfluoroethers, fluoroacrylates, fluorosilicones, and combinationsthereof.
 5. The precursor material of claim 1, wherein saidlow-surface-energy polymer is a siloxane.
 6. The precursor material ofclaim 1, wherein said hygroscopic material is selected from the groupconsisting of poly(acrylic acid), poly(ethylene glycol),poly(2-hydroxyethyl methacrylate), poly(vinyl imidazole),poly(2-methyl-2-oxazoline), poly(2-ethyl-2-oxazoline),poly(vinylpyrolidone), cellulose, modified cellulose, carboxymethylcellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, methylcellulose, hydrogels, PEG diacryalate, monoacrylate, and combinationsthereof.
 7. The precursor material of claim 1, said precursor materialfurther comprising a nanoporous material within at least some of saidinclusions, wherein said nanopores have an average pore diameter lessthan 100 nanometers, and wherein said nanoporous material optionallycomprises a hydrophobic component and/or optionally is surface-modifiedto increase hydrophobicity.
 8. The precursor material of claim 7,wherein said nanoporous material is selected from the group consistingof silica, alumina, silicates, aluminosilicates, carbonates, carbon, andcombinations thereof.